I use mine (prusa i3 reprap) quite a bit; one of my other hobbies is photography (including using a lot of retro lenses and other gear at times) which involves a lot of "attaching this to that", which having a 3D printer is pretty much ideal for.
I've made various lens to body adapters (mostly old legacy manual focus lenses to my Sony A7 [E-Mount]), custom lens caps for odd-sized lenses, custom-sized extension tubes, etc. Just today I 3D printed a custom mount to physically attach a generic ("Neewer" brand) hotshoe flash that I rewired into my Fujifilm Instax 210 instant camera to replace the shitty direct-flash mechanism that the camera ships with so I can use the camera indoors and get decent exposures.
But, yeah, if you can't think of anything you would use a 3D printer to build (if you had one), it is likely to be collecting dust within a couple of weeks after you get it.
That sounds great until you realize you miss out on the economies of scale that injection molding provides.
A toy soldier that costs the better part of a dollar in materials, or one that costs fractions of a penny? Which one is going to provide better margins?
